The present invention relates to systems for energy storage in batteries or for generating energy in fuel cells batteries employing static voltage inverters for converting electric energy stored or generated in batteries from DC to AC when powering electrical loads.
Static voltage inverters for transforming DC electric power in an AC electric power at a certain voltage (for example 240 VAC) are commonly used in renewable power source plants, in “load-leveling” plants, in electric vehicles and the like, using storage batteries or primary batteries of fuel cells for generating energy to power AC loads.
The possibility of converting the DC voltage of batteries of elementary cells, whether they are storage or secondary cells or primary cells such as fuel cells, to an AC voltage of voltage and frequency substantially similar to those of standard power distribution grids, is an important requisite for obvious compatibility reasons and often absolutely necessary in the case of emergency plants (UPS) that must intervene to maintain essential services in function in case of blackouts, and more generally for “grid connectable” systems.
The functional diagram of a system of rechargeable storage batteries is shown in FIG. 1, that in the example shown are vanadium redox flow batteries.
As will be obvious to any skilled person, the inverter depicted in the diagram of FIG. 1 will be the same from a functional point of view whether the DC source is a storage battery of any kind or alternatively a battery of fuel cells.
The use of storage batteries is practically absolutely necessary in stand alone photovoltaic (solar) panels systems not connected to any power distribution grid. Redox flow batteries are much more convenient than other types of storage batteries. Among redox flow batteries, all vanadium batteries, i.e. batteries that employ a vanadium-vanadium redox couple in the negative electrolyte as well as in the positive electrolyte, are particularly advantageous.
Performances of a storage plant employing vanadium redox flow batteries are reported and analyzed in the article: “Evaluation of control maintaining electric power quality by use of rechargeable battery system”, by Daiichi Kaisuda and Tetsuo Sasaki IEEE 2000.
There is a wealth of literature on redox flow batteries and in particular about vanadium redox flow batteries. Therefore, a detailed description of the peculiarities and advantages of such batteries in respect to other types of batteries does not seem necessary in order to fully describe the present invention.
Among many advantages of the redox flow batteries, it is worth remarking though their suitability to being charged even at different charging voltages. To accomplish this, intermediate taps of the electric chain, constituted by the elementary cells in electrical series that constitute the battery, may be used. Depending on the voltage of the available source, most appropriate taps are selected for coupling to the recharging voltage an appropriate number of cells. This is possible because, differently from other types of storage batteries, in redox flow battery systems energy is stored in the electrolytes that circulate through the cells and that are contained in two separated tanks. The battery represents exclusively the electrochemical device in which electric energy transforms in chemical energy and vice versa, and the electrodes do not undergo any chemical transformation during charge and discharge processes.
On the other hand, photovoltaic panels output an electric current when they are irradiated by a light source that may drive an electric load, however if a counter voltage exceeding a certain value is applied on the photovoltaic panel the output current becomes null.
The voltage-current characteristic of a photovoltaic panel has a typical “knee-shaped” curve, the zone enclosed between the curve and the x-axis (voltage) and the y-axis (current) represents the zone in which electric power may be extracted from the panel.
From the above considerations, it is evident that when the output voltage is close to the point of intersection between the voltage-current characteristic and the x-axis, the delivered power (represented by the area of the rectangle with its longest side parallel to the x-axis) becomes very small. Similarly, the available power becomes small also when the output voltage becomes very small.
In practice there is a value of output voltage, within the functioning zone of the characteristic, for which the area of the corresponding rectangle in the voltage-current domain is maximum, i.e. the panel outputs the maximum power that can be extracted.
Such maximum power value varies rapidly when irradiation conditions change. Indeed, the voltage-current characteristic of a solar panel changes in function of the irradiation parameter, practically generating voltage-current curves more or less concentric to each other, in function of the irradiation parameter.
Because this voltage-current characteristic of a photovoltaic panel, the maximum power that can be extracted from the photovoltaic panel varies rapidly with the varying of the conditions of irradiation.
Being obviously desirable to exploit to the fullest extent the available power for charging the storage battery or batteries, this is made possible in a very convenient manner by employing a redox flow battery provided with a plurality of intermediate voltage taps. In function of the current irradiation conditions, one terminal of the photovoltaic panel is automatically switched on the most appropriate intermediate voltage tap such that the battery voltage be as near as possible to the peak voltage of the voltage-current characteristic of the photovoltaic panel at the current irradiation conditions.
There are automatic switching devices that implement such a function, commonly known by the acronym MPPT, for Maximum Power Point Tracker, that optimize the charging conditions of the storage redox flow battery from photovoltaic panel current sources.
It is evident that such a system is much more convenient than using a DC-DC converter for absorbing energy from the photovoltaic panel, at a voltage most appropriate to the irradiation conditions and to boost or reduce it to a certain pre-established regulated output voltage suitable to charge the storage battery or batteries.
Even as far as delivering power to electric loads of different kind is concerned, redox flow batteries have the advantage of being able to support the delivering of current to a load, from a certain number of elementary cells and thus at a corresponding DC output voltage even several order of magnitude greater than the current being delivered by another group of cells, belonging the same battery or to a different battery, at the same output voltage or at a different output voltage, through distinct pairs of intermediate taps, by virtue of the above mentioned peculiar characteristics of redox flow batteries.
Therefore, a battery may be looked at, under certain functional terms, as an electric autotransformer or an electric transformer, wherein there are elementary cells electrically in series of coils and wherein chemical energy is in play instead of magnetic energy.
In view of the particular relevance that redox flow batteries have in many important applications, the ensuing description will refer to redox flow battery systems, notwithstanding the fact that the same considerations will hold even in the case in which batteries of different type are used instead of redox flow batteries, and in particular in the case of batteries of fuel cells.
The choice of the static voltage inverter strongly influences the overall energy conversion efficiency of the system when exploiting the energy storage or generation capacity of batteries of elementary cells.
Usually the efficiency of inverters ranges from a maximum of about 94% at full load and decreases progressively down to about 60% at low load levels.
As it is well known, static voltage inverters employ power switches, such as thyristors (SCR), bipolar junction transistors (BJT), insulated gate bipolar transistor (IGBT) or field effect transistors (MOS), functioning as static switches on DC networks, for realizing devices that switch periodically the connections between the supply rails and the load inverting the polarity. In this way the load is supplied with an alternating voltage, whose frequency depends from the switching frequency of the power switches. The alternating voltage is generally a square wave voltage, whose amplitude is substantially equal to the DC voltage of the voltage source constituted by the battery and by appropriate circuits and by adding output filters it is possible to obtain an almost sinusoidal output voltage.
There are many well known types of power inverters, each well discussed in the pertinent literature.
The “phase opposition inverter” and the “bridge inverter” are well known circuits commonly employing load inductors and/or transformers and turn off capacitors for coordinately and alternately turning off distinct power switches, commonly constituted by thyristors.
Inverters with a sinusoidal output are known, from which an output voltage wave approximating a sinusoid may be obtained. Such an inverter may be the same bridge inverter employing inductors or transformers is in series with a capacitor. By turning the equivalent load inductance and other eventual impedances in series with the capacitor for a desired output frequency, it is possible to produce an almost sinusoidal output wave.
When batteries are the electric energy sources, the regulation of an AC voltage output by the inverter must be realized by means of the same systems that are normally used for regulating AC voltages, such as sliding contact autotransformers, saturating core magnetic regulators and the like.
Independently from the type, the plant of energy storage or of generation in a plurality of elementary cells employs multicell batteries, i.e. batteries that are constituted by a great number of elementary cells electrically in series. Of course the plant may employ even a plurality of multicell batteries, electrically connected according to common series-parallel schemes, for ensuring the desired capacity of delivering electric power at a certain supply voltage.
When recharging storage batteries, the scheme of connection to the recharging DC source may be the same or more often is different from the scheme that is implemented during a discharge phase, and the different schemes may be implemented automatically by means of configuration switches and/or path selectors.
The driving in switched mode of the transformer, i.e. of the inductance of the circuit of a classic inverter, determines functioning conditions that are particularly burdensome for power switches, whether they are SCR, BJT or MOS. They must be precisely controlled to prevent failures. These functions are normally performed by appropriate turn on control and thermal protection circuits.
The transformer and/or the inductors commonly used in inverters dissipate power besides being relatively expensive because of peculiar features that often they must possess.
When to output a substantially sinusoidal waveform voltage is required, the cost of the inverter becomes considerably greater because of the greater complexity necessary for ensuring a precise and sufficiently constant frequency and eventually also for filtering the output voltage.